1. Field of the Invention
The field of the present invention relates to varistor compositions.
2. Description of Related Art
Zinc oxide varistors are ceramic semiconductor devices based on zinc oxide. They have highly non-linear current/voltage characteristics, similar to back-to-back Zener diodes, but with much greater current and energy handling capabilities. Varistors are produced by a ceramic sintering process which gives rise to a structure consisting of conductive zinc oxide grains surrounded by electrically insulating barriers. These barriers are attributed to trap states at grain boundaries induced by additive elements such as bismuth, cobalt, praseodymium, manganese and so forth.
The electrical characteristics of a metal oxide varistor, fabricated from zinc oxide, are related to the bulk of the device. Each zinc oxide grain of the ceramic acts as if it has a semiconductor junction at the grain boundary. The non-linear electrical behavior occurs at the boundary of each semi conducting zinc oxide grain. Accordingly, the varistor can be considered as a multi-junction device, composed of many series and parallel connections of grain boundaries. The device behavior may be analyzed with respect to the details of the ceramic microstructure. Mean grain size and grain size distribution play a major role in electrical behavior.
Fabrication of zinc oxide varistors has traditionally followed standard ceramic techniques. The zinc oxide and other constituents are mixed, for example by milling in a ball mill, and are then spray dried. The mixed powder is then pressed to the desired shape, typically tablets or pellets. The resulting tablets or pellets are sintered at high temperature, typically 1,000xc2x0 to 1,400xc2x0 C. The sintered devices are then provided with electrodes, typically using a fired silver contact. The behavior of the device is not affected by the configuration of the electrodes or their basic composition. Leads are then attached by solder and the finished device may be encapsulated in a polymeric material to meet specified mounting and performance requirements.
In the device thus fabricated, the bulk of the varistor between its contact or electrode layers thus consists primarily of zinc oxide grains of a predetermined average grain size, yielding a specific resistivity per unit of thickness dimension. In designing a varistor for a given nominal varistor voltage, it is therefore basically a matter of selecting a device thickness such that the appropriate number of grains is in series between the electrodes. The voltage gradient of the varistor material, in terms of volts per unit of thickness dimension, can be controlled by varying the composition and manufacturing conditions of the varistor. Altering the composition of the metal oxide additives enables the grain size to be changed for this purpose. In practice, the voltage drop per grain boundary junction is approximately constant and does not vary greatly for grains of different sizes. Accordingly, varistor voltage is primarily determined by the thickness of the material and the size of the grains.
The construction and performance of varistors is discussed inter alia in xe2x80x9cZinc Oxide Varistorsxe2x80x94A Reviewxe2x80x9d by L. M. Levinson and H. R. Philipp, Ceramic Bulletin, Volume 65, No. 4 (1966), which article may be referred to for further detail.
A multiplicity of specific varistor compositions are known and described, inter alia, in the following patent specifications: U.S. Pat. No. 3,598,763; U.S. Pat. No. 3,663,458; U.S. Pat. No. 3,863,193; U.S. Pat. No. 4,045,374 and GB 1,478,772. Methods of manufacturing varistors are described, inter alia, in U.S. Pat. Nos. 3,863,193 and 4,148,135.
It is an object of the present invention to provide improved compositions of material for use in producing varistors. It is a further object of the invention to provide suspensions of zinc oxide based materials in solvents for use in producing varistors using a wet screen printing method.
The present invention is especially directed to compositions facilitating the manufacture of multilayer varistors. While it has generally been accepted that a multilayer varistor would have a number of advantages as compared with the equivalent radial product, manufacturing problems have hitherto prevented any widespread move towards multilayer varistors.
The advantages of multilayer construction as applied to varistors include compact size for equivalent electrical characteristics, as compared with a conventional radial device. Multilayer varistors may also be completely symmetrical, fully passivated and have good IV characteristics. As against this, possible disadvantages include relatively high capacitance and potential reactions between the ceramic and the internal electrodes, especially the interaction of palladium and bismuth complexes.
The present invention is further especially directed to compositions for use in the manufacture of multilayer varistors using printing techniques. A method of manufacturing a multilayer varistor by a sequence of printing operations forms a subject of a co-pending patent application by the present applicants. In a particular manufacturing method disclosed in said co-pending application, both the ceramic layers and the electrode patterns are successively screen printed. A particular advantage of screen printing as compared with other manufacturing technologies for multilayer systems is that the preparation of a thin sheet material as a preliminary step in the process may be obviated, by virtue of the direct laying down of the ceramic material and the electrode pattern in a succession of printing steps. The printing process therefore provides a more convenient end expeditious method of manufacturing multilayer products, and in particular varistors, than methods involving the preliminary step of fabricating sheets or panels of ceramic and electrode material for interleaving in a later production operation. However, the successful implementation of a printing manufacturing technology for multilayer varistors requires the provision of compositions suited to this production process and capable of cooperating with solvent materials to provide the required ceramic and electrode inks.
It is therefore also a particular object of the present invention to provide inks suited to screen printing manufacturing technologies for multilayer varistors. Powder compositions suitable for incorporation in such inks form the subject of a co-pending application of the present applicants, as indicated above, and incorporated herein by reference.
According to the invention, there is provided a composition material for use in manufacturing a varistor comprising:
(a) zinc oxide;
(b) a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boric acid, chromium oxide, cobalt oxide, manganese oxide and tin oxide;
(c) at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide;
(d) an organic solvent carrier;
(e) an organic viscosity-influencing additive; and
(f) an organic binder.
The above-mentioned plurality of ceramic structure influencing additives may include at least bismuth oxide, cobalt oxide and manganese oxide. Preferably the composition material comprises at least one electrical performance influencing additive selected from the group consisting of at least aluminum nitrate and silver oxide. The composition material may also comprise nickel oxide as a further additive and magnesium hydroxide as an additional additive.
In another aspect, the invention provides a composition material for use in manufacturing a varistor comprising:
(a) 94 to 98 mole percent of zinc oxide;
(b) 1 to 4 mole percent of a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, cobalt oxide, manganese oxide and tin oxide;
(c) 0.1 to 1.6 mole percent of at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide;
(d) an organic solvent carrier;
(e) an organic viscosity-influencing additive; and
(f) an organic binder.
The composition material may comprise 0.002 to 0.01 mole percent of at least one electrical performance influencing additive selected from the group consisting of at least aluminum nitrate and silver oxide. The composition material may also comprise 0.6 to 1.1 mole percent of nickel oxide as a further additive, and at least 0.4 mole percent of magnesium oxide as an additional additive.
Preferably, the inorganic constituents of the composition material are in granular or powder form and the average grain size of the granular or powder form material is less than approximately 2 microns.
Suitably, the relative proportions of the organic constituents are selected so that the mixture of the organic and inorganic constituents of the composition material is in the form of a suspension.
The invention also provides a printing ink comprising a composition material as defined hereinabove, as well as a method of producing a composition material for use in manufacturing a varistor comprising the steps of:
(i) calcining a granular or powder form composition material comprising:
(a) zinc oxide;
(b) a plurality of ceramic structure influencing additives selected from the group consisting of at least bismuth oxide, boron oxide, chromium oxide, cobalt oxide, manganese oxide and tin oxide; and
(c) at least one grain growth influencing additive selected from the group consisting of at least antimony oxide, silicon dioxide and titanium dioxide; and
(ii) mixing the calcined composition material with an organic solvent carrier, an organic viscosity-influencing additive, and an organic binder.
Preferably the above-indicated additive is added for a first stage of the mixing step, and the binder is added following the first stage of the mixing step, the binder being mixed with other constituents of the material in a second stage of the mixing step. The first stage of the mixing step suitably comprises a milling operation.